Rat lungs are incompletely developed at birth. Large conducting airways with columnar epithelium which end in thick-walled saccules containing a double pulmonary capillary network are transformed postnatally into definitive alveoli by outgrowth of secondary septa from primary septa, forming angles recognizable as alveolar "corners". Terminal bronchioles are converted into respiratory bronchioles by outpouching of the walls and close apposition of capillary loops to the epithelium. In mature epithelium type II cells are usually found at alveolar corners while in fetal and early neonatal rat lungs (1-4), type II cells are uniformly distributed. The ratio between the two cell types in mature lungs is constant. This application seeks to address the questions: What factors regulate the localization of type II cells in the alveoli? What mechanisms regulate the restoration of a constant numerical ratio between the cell types following injury? When are the epithelial cell patterns established: Our hypothesis is that lung basement membrane (BM) contains local specialized areas which regulate the differentiation and location of type I or type II cells. Because of its close apposition to the lung epithelium, the capillary endothelium through its BM, may be an important regulator of the type II cell transformation into type I phenotype. We postulate that the distribution pattern of BM microdomains is established during the neonatal period and that normal repair results in re- establishment of the microdomains. Hyperoxic lung injury during the neonatal period development may delay or prevent establishment of a normal pattern of lung epithelial cell localization. To test this hypothesis, we will culture isolated neonatal type II cells and a type II cell-derived cell line (LM5) on normal and rat lung matrices isolated from rats exposed to hyperoxic lung injury. The differentiation of the cultured cells into either type I or type II cells and their localization will be evaluated by morphometry and immunohistocytochemistry. We plan to identify the BM component(s) which may mediate this differentiation and the cell-matrix receptors which recognize the BM signals.